1. Field of the Invention
The present invention relates to ballasts for fluorescent lamps, particularly of a type having built-in means for preventing ground-fault currents from the ballast output.
2. General Background and Rationale
With most fluorescent lighting fixtures, the voltages required at the sockets to start the fluorescent lamps are so high as potentially to constitute a substantial electric shock hazard to persons having to service such fixtures. To mitigate this hazard, whenever socket voltages exceed certain levels, protective measures have to be provided.
The essential shock hazard problem associated with a fluorescent lighting fixture relates to the situation where a person, who may be in contact with earth ground, is holding onto one end of a fluorescent lamp while the other end of the lamp is inserted into its socket.
Consequently, by using lamp sockets of a type that provides for disconnection of the socket voltage whenever a lamp is removed, acceptably safe operation will result. Alternatively, electrical isolation can be provided between ground and the ballast output to the lamp sockets.
In most applications of inverter-type ballasts, the resulting socket voltages are high enough to require protective measures; and the only presently available commercially practicable solution is that of using an isolation transformer to provide electrical isolation between the power line input (ground) and the ballast output to the lamp sockets. While this solution is indeed safety-wise acceptable, it involves substantial penalties in terms of ballast cost, size and weight, as well as in overall ballast efficiency.
Of course, circuit-interrupting sockets could be used; but that solution would require non-standard and substantially more costly lamp sockets in addition to extensive added wiring within the fixture. Thus, for electronic ballasts the use of circuit-interrupting lamp sockets would constitute an even less attractive solution than that of using an isolation transformer.
Based on the background outlined above, subject invention relates to a very cost-effective electronic means of providing a function substantially equivalent to that provided by circuit-interrupting lamp sockets.